Biocatalytic recycling of polyethylene terephthalate plastic

Abstract

The global production of plastics made from non-renewable fossil feedstocks has grown more than 20-fold since 1964. While more than eight billion tons of plastics have been produced until today, only a small fraction is currently collected for recycling and large amounts of plastic waste are ending up in landfills and in the oceans. Pollution caused by accumulating plastic waste in the environment has become worldwide a serious problem. Synthetic polyesters such as polyethylene terephthalate (PET) have widespread use in food packaging materials, beverage bottles, coatings and fibres. Recently, it has been shown that post-consumer PET can be hydrolysed by microbial enzymes at mild reaction conditions in aqueous media. In a circular plastics economy, the resulting monomers can be recovered and re-used to manufacture PET products or other chemicals without depleting fossil feedstocks and damaging the environment. The enzymatic degradation of post-consumer plastics thereby represents an innovative, environmentally benign and sustainable alternative to conventional recycling processes. By the construction of powerful biocatalysts employing protein engineering techniques, a biocatalytic recycling of PET can be further developed towards industrial applications. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Keywords: biocatalysis; circular economy; plastics; polyethylene terephthalate; recycling.

Figure 1.

Global production, use and fate of polymer resins, synthetic fibres and additives 1950–2015 (in million metric tons) [1]. (Online version in colour.)

Figure 2.

Products obtained from amorphous PET films degraded by a polyester hydrolase. TPA, terephthalic acid; EG, ethylene glycol; MHET, mono-(2-hydroxyethyl) terephthalate; BHET, bis-(2-hydroxyethyl) terephthalate [7].

Figure 3.

Surface representation of a polyester hydrolase with a model compound composed of two repeating units of PET polymer (blue carbon atoms) docked within the substrate groove. Three independently docked conformations are shown in stick representation. The position of the active serine is marked on the surface in red [17]. The docking provides information on the relevant amino acids at the active site of the enzyme and the construction of superior variants. (Online version in colour.)

Figure 4.

Biocatalytic PET recycling with a polyester hydrolase in an ultrafiltration membrane reactor [24]. (Online version in colour.)